package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;

/**
 * A DocIdSet contains a set of doc ids. Implementing classes must
 * only implement {@link #iterator} to provide access to the set. 
 */
public abstract class DocIdSet {

    /** An empty {@code DocIdSet} instance for easy use, e.g. in Filters that hit no documents. */
    public static final DocIdSet EMPTY_DOCIDSET = new DocIdSet() {

        private final DocIdSetIterator iterator = new DocIdSetIterator() {
            @Override
            public int advance(int target) throws IOException {
                return NO_MORE_DOCS;
            }

            @Override
            public int docID() {
                return NO_MORE_DOCS;
            }

            @Override
            public int nextDoc() throws IOException {
                return NO_MORE_DOCS;
            }
        };

        @Override
        public DocIdSetIterator iterator() {
            return iterator;
        }

        @Override
        public boolean isCacheable() {
            return true;
        }
    };

    /** Provides a {@link DocIdSetIterator} to access the set.
     * This implementation can return <code>null</code> or
     * <code>{@linkplain #EMPTY_DOCIDSET}.iterator()</code> if there
     * are no docs that match. */
    public abstract DocIdSetIterator iterator() throws IOException;

    /**
     * This method is a hint for {@link CachingWrapperFilter}, if this <code>DocIdSet</code>
     * should be cached without copying it into a BitSet. The default is to return
     * <code>false</code>. If you have an own <code>DocIdSet</code> implementation
     * that does its iteration very effective and fast without doing disk I/O,
     * override this method and return <code>true</here>.
     */
    public boolean isCacheable() {
        return false;
    }
}
